The Iowa caucuses mean different things for different candidates. Of course, Barack Obama, Hillary Clinton and John Edwards, among the Democrats, and Mike Huckabee and Mitt Romney, among the Republicans, are vying for a win in Iowa tonight.

But several subprimaries are going on, as well: Obama vs. Edwards for the position of chief challenger to Clinton, John McCain vs. Rudy Giuliani for the right to wear the “moderate” Republican mantle and Fred Thompson vs. oblivion for the right to stay in the race.

So here’s the scorecard to use in keeping track of what each candidate needs to get from Iowa:

Clinton:

If she scores a decisive triumph, the race for the nomination is almost over. A victory here would likely propel her to a win in New Hampshire, and the nomination would be hers. But she doesn’t need to win. Her national base is so strong that she just has to stop anyone else from winning.

If no clear winner emerges, but the results show, instead, a three-way tie with Obama and Edwards, or a two-way tie between herself and either of her challengers, she comes out the winner.

But if she clearly loses by a good margin or finishes third, she has blown a major opportunity and is in for a long cold winter of primaries. She won’t be knocked out in Iowa, no matter what, but she could be knocked down.

Obama:

He’s got to win in Iowa. He’s so far behind Clinton in the national standings that he needs a decisive victory to give him the momentum to prevail in New Hampshire and to compete in Florida and on Super Tuesday. He also needs to leave the pesky Edwards far behind so he can consolidate the anti-Clinton vote behind his candidacy.

Edwards:

His immediate need is to finish close to or ahead of Obama so he can show that a vote for him isn’t wasted. With pro- and anti-Clinton sentiment so strong, Edwards risks being excluded as an also-ran if he doesn’t make it. He also needs Clinton not to win decisively so that the race stays alive. He has a decent shot in New Hampshire if he can stay in the race and make sure there still is one.

Huckabee:

Win or die are his choices. The Huck-a-boom will be right in the ancient history books with the Howard Dean surge in September 2003. But even if Huckabee wins in Iowa, he’ll probably lose in New Hampshire. Then his candidacy will come back to another game-set-match point in Michigan the following week.

Romney:

He doesn’t have to win, place or show. He’s got a big checkbook so he can survive any kind of showing and stay in the game. But a defeat in Iowa might make him vulnerable to McCain in New Hampshire. A loss in the first two states would cost him Michigan, and he would limp into Super Tuesday with only a checkbook to protect him. Only.

McCain:

He’s got to finish third – or, in other words, beat Rudy. If he does, he has a good shot at winning New Hampshire and getting back into the game. If he doesn’t, Romney will win New Hampshire and McCain will be out of the race. Huckabee has to hope McCain does finish third, so Romney doesn’t win New Hampshire and, therefore, doesn’t win Michigan. Got it?

Giuliani:

The Republican front-runner is in a parallel situation with Clinton. He won’t be knocked out no matter how badly he does. But finishing below McCain would mean that he has to split the moderate vote with the charismatic Arizona senator and could weaken his chances in Florida and on Super Tuesday.

Giuliani can lose Iowa, New Hampshire, Michigan, Nevada and South Carolina and still survive to compete in the big states that follow. But he lost a golden chance to avoid a fight by winning in Iowa.

And, if Clinton wins big in Iowa, it will help McCain and hurt Romney in New Hampshire. Why? All the independents who would’ve voted for or against Clinton in New Hampshire will pile into the Republican primary and may boost McCain to victory (if he survives Iowa).

And, by the same token, Giuliani needs Hillary to win in the early rounds so he can draw independents into the Republican primary to vote for him rather than the religious-right crowd.

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